Clearly, the snake's brain is far more primitive than our own so will not include "emotion" as we understand it, but there will still be identifiable reactions to different situations, which can be associated with differing states of mind.

For example, the "essing up" which my boa constrictor will sometimes do is a defensive/offensive stance, and suggests that it might strike, and arboreals will react differently from ground dwellers to being placed around someone's neck, whereas a snake feeling "at ease" will be much more relaxed, perhaps rubbing against a person simply for warmth. (Speculation as to how the snake perceives people under different conditions is beyond the scope of what I'm asking but would, I'm sure, be a fruitful area for research)

What other identified behaviours are there, which can serve to convey a snake's likely next actions?

3 Answers
3

There are roughly three kinds of moods you need to be able to tell apart in snakes:

Hungry / looking for food

Scared / pissed off

Relaxed / exploring around

How exactly these moods look differs immensely between species and even specimens. Even snakes can have wildly varying personalities.

'Essing up'

For example, the "essing up" which my boa constrictor will sometimes do is a defensive/offensive stance, and suggests that it might strike,

Not necessarily. Sure, having this 'S'-shape in their body will mean that if they strike, they can suddenly cover a bigger distance, but that doesn't mean that always if they coil up like this they want to strike, and neither do they always coil up before striking. So all in all, it is a bad indicator.

My Boa will just idly hang down her branch. This kinda looks like an ambush position, but that does not reflect her attitude when the cage is opened, so I'm going to chalk it up as idle.

My Ball python will also do this as a part of their defensive posture, even when they have no intention of striking. Having their head closer to their body also means that it is easier for them to protect their head.

All in all, I'd say it isn't the best indicator for their mood.

Tongue-flicking

Snakes have very bad eye-sight. So they rely very much on scent as sense. For this they use their iconic tongue-flicking. Each time they do that, they smell their surroundings using an organ in the top of their mouth.

The way they use this is very indicative for their mood. If they are relaxed and moving around, they'll just flick lazily every now and then to take in their surroundings, to check if there is anything interesting somewhere.

When they have to be more aware of their surroundings, for example if they feel that they are in danger, or that there is food somewhere nearby, they will be flicking a lot more. Mostly I would describe it as much more agitated. Many short flicks.

At this point, it is still hard to distinguish food-mode from angry-mode, but that will come with:

Movement around them

This is about the situation when you are near the snake, with the cage open for example.

How does the snake react to your movements.

If they are relaxed, they might very well just ignore you. They'll go about their happy business, whether that is sleeping (likely) or exploring (depends a bit on species).

As you can see above, she is just moving around, minding her own business, tongue-flicking every now and then and ignoring my movement with the camera :)

However, if they are agitated, they will be much more inclined to follow any movement around them. This might be very subtle, like tracking the movement just with an eye, or they might just follow it with their head. This is a point where it is easier to tell defensiveness apart from a food-response.

Snakes that feel threatened are not likely to give chase. Of course it depends on the species and disposition of the snake, but if they feel threatened, most of all they just want to be left alone. Getting away is always a win.

However, if the threat doesn't go away, they might bite. Some species decide to bite very quickly, some species are known for being general assholes, but most commonly kept snakes are pretty laid back. For example, my Ball python is 10 years old now, and in the 8 years I have had him, he has struck only twice out of self-defense.

In contrast, if the movement has sparked their interest because they think it is food, their reaction (on top of tongue-flicking) will be more forward. They will be much more inclined to follow the movement in a forward direction, inching towards it.

This can be seen in the gif below. This is my Boa's reaction to food in its enclosure. You can see their immediate attention. You can see her tongue-flicking and essing up. But the two most important things to take away here are that she moves her head towards any movement and actively follows it, along with creaping forward. This shows a great deal of interest, and her going forward towards the movement is a great tell that this is not a defensive posture.

Flighty behaviour

When you are handling your snake and they are not comfortable, this might also show itself in the snake suddenly trying to move away quickly, as if they are trying to flee. This should really speak for itself.

Experience

A lot of this is down to experience and really relies on getting to know your snake. Some snakes are really keen on food and will turn into food-mode on a whim. It is important to know how to recognise this and when you only just have the snake, it is never a bad idea to err on the safe side.

If you do it right, you should never really be the target of their food-mood. First you get to know what triggers it, then you make sure you prevent that from happening whenever it is not time for food.

For many snakes, one such trigger will be the scent of their food. This is generally easy to handle by making sure food items are only nearby when it's actually feeding time and by washing your hands afterwards.

Similarly, after a while you'll learn to notice when your snake is not comfortable and be able know when to leave them alone.

We must be careful to make assumptions. Emotions like being at ease or comfortable are relative to the eye of the beholder. How tight do you for example think a tree boa wraps itself around a tree? (I dunno?) and how tight do you think a constrictor coils around its prey? Are these agressive behaviours? Are they normal? Are they conformable behaviours? Who knows?

I can only talk from experience here with my own snakes.

Mating behaviour sometimes goes in hand with tail tip movements.

Female egg laying often goes with burrowing behaviour.

And the obvious one we all know is fear and/or aggression can be identified through hissing, striking out, and fleeing.

This could be useful. It would be great if you had a little more than your own experience. If you find more scientific resources to back up your own experience, it would be great if you added them to your answer.
– Vixen PopuliMar 20 '16 at 7:51

That is the problem though. I don't think the habits or snake behaviour is going to be high on the priority list of any biologist other than what someone maybe on BBC Planet Earth noticed during one of his shoots ;) I studied basic animal Biology and Physiology on tertiary level and I don't recall any specific reference to behaviour other than what barers snakes are, poisons they have and types of methods they use to kill their prey. This opinion might differ from higher graduates though.
– Renier DelportMar 20 '16 at 23:28

In general, if a reptile doesn't like what you are doing, whether that
is handling him, petting him, or anything else, he will try to get
away from you or possibly even bite. Signs that something in his
environment is making your reptile feel insecure, uncomfortable, or in
pain include:

-Breathing heavily with lots of noticeable movement in the rib cage area

-Fussiness, squirming, and hissing in a usually tame reptile

-Trying to climb you or get away

-Drawing away when you go to pick him up

-Striking and biting

-Wraps himself very tightly around your arm, leg, or torso

-Draws back with his head and neck lifted off the ground (an S-curve) Buzzes his tail

-Thrashes around in your hands

-Strikes at you (full strikes or false strikes)

I read this article once, which made me shocked (I first saw it on facebook)

This short story, which has been shared widely across Facebook by user
Chris Planer, is something to think about.

There once was a woman who had a pet snake that she loved very much.
The snake was about 7 feet long and one day it just stopped eating.
After several weeks of trying everything she could to get the snake to
eat, the woman took it to the vet.

The woman explained the situation to the vet and he asked her, “Has
your snake been sleeping with you at night or snuggling really close
and stretching himself out?”

The woman replied, “Yes he’s been doing it everyday and it makes me so
sad that I can’t help him feel better.”

The vet says “Ma’am your snake is not sick, he has been preparing to
eat you. He’s been sizing you up everyday so he knows how big he has
to be, and not eating so he has enough room to digest you.

Moral of the story: You’ve gotta recognise the snakes out there. Just
because they seem close to you and sleep in your bed, it doesn’t mean
their intentions are good.

If this isn’t some food for thought – no pun intended – then we’re not sure what is.